1. Field of the Invention
The present invention relates to a camera and/or camera zoom lens system, and more particularly, though not exclusively, to a camera zoom lens and/or camera having a function for presetting focus to any distance.
2. Description of the Related Art
In recent years, cameras with autofocus and zoom lens have become widespread. FIG. 8 illustrates a block diagram of a conventional example of a digital camera with a zoom lens.
An optical system 1 is made up of a four-group rear focus zoom lens (hereinafter called “RFZ lens”) construction consisting of four lens groups. The RFZ lens 1 consists of a first lens group (hereinafter called “front lens”) 101 as a fixed lens group, a second lens group (hereinafter called “zooming lens”) 102 as a moving lens group having a magnification varying function, a third lens group (hereinafter called “afocal lens”) 103 as another fixed lens group, and a fourth lens group (hereinafter called “focus lens”) 104 as another moving lens group having both a focusing function and a compensator function for reducing a shift in the position of an imaging surface due to a change in magnification.
Although each lens group actually consists of one or more lenses, the number of elements in each lens group is not particularly limited.
Reference numeral 2 designates a photoelectric conversion device such as a CCD, reference numeral 3 designates an aperture mechanism for regulating the amount of light incident on the photoelectric conversion device 2, and reference numeral 4 designates aperture driving device through which the aperture mechanism 3 is controlled by a control circuit 7 to maintain a constant amount of light incident on the photoelectric conversion device 2. Reference numeral 5 designates an aperture and lens position detecting device, and reference numeral 6 designates a detection circuit for detecting the output of the aperture and lens position detecting device 5 and outputting it to the control circuit 7.
Reference numerals 8 and 9 designate lens driving devices, such as step motors, for driving the moving lens groups 102 and 104, respectively, and reference numerals 10 and 11 designate motor drivers for driving the lens driving devices 8 and 9, respectively. The control circuit 7 controls the lens driving devices 8 and 9 to drive the lens groups 102 and 104 based on their position detection signals from the aperture and lens position detecting device 5.
Reference numeral 12 designates an amplifier for amplifying the output of the photoelectric conversion device 2, reference numeral 13 designates a process circuit for converting the output signal to a signal such as an NTSC picture signal while generating an AF signal for automatic focus (AF), and reference numeral 14 designates an AF device for performing an AF operation according to the AF signal from the process circuit. Although a hill-climbing method is proposed as an AF method, since it is known in the art, its detailed description will be omitted (see, for example, Japanese Patent Application Laid-Open No. 62-103616 (1987)).
FIG. 3 illustrates a graph showing information on the position of the focus lens 104 relative to the position of the zooming lens 102 illustrated in FIG. 8. For example, suppose that zooming is done in such a condition that a subject located at a distant (e.g., at infinity) position is in focus. In this case, in order to maintain focus on the subject at all times during zooming, it is necessary to move the focus lens 104 according to the position of the zooming lens 102 in such a manner that the focus lens 104 will move to follow the track of cam movement, or cam trajectory (e.g., the infinity curve (8) illustrated in FIG. 3).
In a rear focus zoom lens system, cam trajectory information as shown in FIG. 3 is prestored in a storage device, so that during zooming the position of the focus lens relative to the zooming lens is determined from the cam trajectory information and controlled to maintain its focus.
FIG. 9 illustrates a flowchart showing the flow of a focus preset function of a zoom lens of a front focus type. When a storage operation is performed, the focus position at the time when the operation is done is stored, while when an execution operation is performed, the focus position is compared with the stored position to drive the focus lens to the stored position. At first, it is determined whether a storage operation is present (S901). If present, the current focus position is stored (S902) and the procedure goes to the next step. If not present, the procedure goes to step S903 to determine whether an execution operation is present. If the execution operation is present, it is determined whether stored information (stored position) is present (S904). If present, the current focus position is compared with the stored position (S905). If the current position does not match the stored position, the direction of driving the focus lens is determined (S906) to drive it (S907) in the direction, and the procedure returns to step S905. These operations are repeated until the current position matches the stored position. If there is no stored position in step S904, the procedure returns to step S901. The stored, focus-preset position is not changed even when the position of the zooming lens changes.
Recently, in order to expand the possibilities of photography, there has been a strong demand for image-taking apparatuses such as cameras having a focus preset function for shifting focus to a predetermined stored position in a given manner. The demand for compact size and excellent AF performance has also been increasing. For these reasons, the rear focus lens design is becoming popular for optical apparatuses.
However, in the conventional rear focus lens, the position of the focus lens, needed to maintain its focus, varies according to the zoom position. Therefore, when performing zooming, the system has to do more than just store the focus position, or it will result in a deviation from the stored position of distance from a subject (camera-to-subject distance).
In other words, in such a rear focus lens construction, the stored, focus-preset data is almost equal to nothing. Even if such data is stored, it can be subject to some constraints (e.g., canceling the stored data whenever a zooming operation is performed). Thus, conventional systems do not use focus preset functions in a rear focus lens configuration.